Datasheet LT8315 (Analog Devices) - 9

LT8315
OPERATION
The LT8315 is a high-voltage current-mode switching
Boundary Mode Operation
regulator designed for the isolated flyback topology. The Boundary mode is a variable frequency, current-mode special problem normally encountered in such circuits is switching scheme. The internal N-channel MOSFET turns that information relating to the output voltage on the isolated on and the inductor current increases until it reaches the secondary side of the transformer must be communicated limit determined by the voltage on the VC pin and the sense to the primary side in order to achieve regulation. This is resistor’s value. After the internal MOSFET turns off, the often performed by opto-isolator circuits, which waste voltage on the tertiary winding rises to the output voltage output power, require extra components that increase the multiplied by the transformer tertiary-to-secondary turns cost and physical size of the power supply, and exhibit ratio. After the current through the output diode falls to trouble due to limited dynamic response, nonlinearity, zero, the voltage on the tertiary winding falls. A boundary unit-to-unit variation, and aging over life. mode detection comparator on the DCM pin detects the The LT8315 does not need an opto-isolator because it negative dV/dt associated with the falling voltage and trig- derives its information about the isolated output voltage gers the sample-and-hold circuit to sample the FB voltage. by examining the flyback pulse waveform appearing on a When the tertiary voltage reaches its minimum and stops tertiary winding on the transformer. The output voltage is falling, the boundary mode comparator turns the internal easily programmed with two resistors. MOSFET back on for minimal switching energy loss. The LT8315 features a boundary mode control method (also Boundary mode operation returns the secondary current called critical conduction mode), where the part operates to zero every cycle, so parasitic resistive voltage drops at the boundary between continuous conduction mode and do not cause load regulation errors. Boundary mode discontinuous conduction mode. Due to boundary mode also allows the use of a smaller transformer compared operation, the output voltage can be determined from the to continuous conduction mode and does not exhibit tertiary winding’s voltage when the secondary current is subharmonic oscillation. almost zero. This method improves load regulation without extra resistors and capacitors.
Discontinuous Conduction Mode Operation
The Block Diagram shows an overall view of the system. As the load gets lighter, the peak switch current decreases. Many of the blocks are similar to those found in traditional Maintaining boundary mode requires the switching fre- switching regulators, including current comparator, internal quency to increase. An excessive switching frequency reference, LDO, logic, timers, and an N-channel MOSFET. increases switching and gate charge losses. To limit these The novel sections include a special sampling error ampli- losses, the LT8315 features an internal oscillator which fier, a temperature compensation circuit, an output current limits the maximum switching frequency to 140kHz. Once regulator, and a depletion-mode startup FET. the switching frequency hits this limit, the part starts to reduce its switching frequency and operates in discontinu-
Depletion Startup FET
ous conduction mode. The LT8315 features an internal depletion mode MOSFET.
Low Ripple Burst Mode Operation
At startup, this transistor charges the INTVCC capacitor so that the LT8315 has power to begin switching. This Unlike traditional flyback converters, the internal MOSFET removes the need for an external bleeder resistor or other has to turn on and off to generate a flyback pulse in order components. to update the sampled output voltage. The duration of a well-formed flyback pulse must exceed the minimum-off time for proper sampling. To this end, a minimum switch turn-off current is necessary to ensure a flyback pulse of sufficient duration. 8315fa For more information www.linear.com/LT8315 9 Document Outline FEATURES APPLICATIONS DESCRIPTION TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS